The following local policy suppresses the username, URL path details, and referrer header from an HTTP request when that request is saved to the 'main' access log:
<Proxy>
log.rewrite."cs(Referer)"[main]("$(cs(Host))") log.suppress."cs(Referer)"[main](no) log.suppress.cs-uri-path[main](yes) log.suppress.cs-username[main](yes)
Please see How do I add CPL to a local policy file on the ProxySG for instructions on working with local policy and CPL.